A resin layer having cholesteric regularity (hereinafter, it will be referred to as “cholesteric resin layer”) has a characteristic of reflecting a circular polarized light which is in a rotational direction that is the same as the direction of helical rotation of cholesteric regularity (hereinafter, the characteristic will be referred to as “selective reflection characteristic”).
A wavelength range which exhibits the selective reflection characteristic depends on the period of cholesteric regularity. By increasing the distribution width of the period of cholesteric regularity, it is possible to increase the width of the wavelength range which exhibits the selective reflection characteristic (hereinafter, it will be referred to as “selective reflection band”).
If it is possible to form a circularly polarized light separating sheet comprising a cholesteric resin layer having the selective reflection band in the visible light wavelength range, of natural incident light, the circularly polarized light separating sheet reflects only the circularly polarized light of a specific wavelength and transmits circularly polarized lights of other wavelength. Reutilization of the reflected light is possible by allowing the same to re-enter to the resin layer using a reflector, etc. Also, it is possible to convert natural light to linearly polarized light with high efficiency by using the circularly polarized light separating sheet in combination with a quarter-wave plate. Furthermore, a high-luminance liquid crystal display device can be obtained by aligning the direction of the linearly polarized light with the transmission direction of an absorption-type polarizer of a liquid crystal display device, comprising polyvinyl alcohol, etc.
To form the cholesteric resin layer having the selective reflection band in the visible light wavelength range, various chiral agents have been studied.
For example, Patent Literature 1 discloses a chiral compound represented by the formula (Z11—Y11-A11-O—CO—O-M11-Y12)r1Xa wherein A11 is a spacer; M11 is a mesogenic group; Y11 and Y12 are chemical bonds or —O—, —S—, —CO—O—, —O—CO—, —O—CO—O—, —CO—N(Ra)— or —N(Ra)—CO—; Xa is r1-valent chiral radical; Ra is hydrogen or C1-C4-alkyl; r1 is from 2 to 6; and Z11 is: a1) at least one of these radicals is a reactive group which can participate in a polyaddition reaction, a2) at least two of these radicals are substituents carrying a reactive group which can participate in a polycondensation reaction, b1) is hydrogen or an unreactive radical so long as condition (a1) or (a2) is satisfied.
Patent Literature 2 discloses a chiral compound represented by the formula: (Z12—Y13-A12-Y14-M12-Y15) r2Xb wherein A12 is a spacer; M12 is a mesogenic group; Y13 to Y15 are chemical bonds or —O—, —S—, —CO—O—, —O—CO—, —O—CO—O—, —CO—NRb— or —NRb—CO—; Rb is hydrogen or C1-C4-alkyl; Xb is r2-valent chiral group; r2 is 2 to 6; and Z12 is: (a3) at least one of these groups is a group with isocyanate, isothiocyanate, cyanate, thiirane, aziridine, carboxyl, hydroxyl or an amino group, (b2) the other group (s) is/are hydrogen or unreactive group.
Patent Literature 3 discloses a compound represented by the formula: (Z13—Y16-[A13]r3-Y17-M13-Y18-)r4Xc wherein A13 is a spacer; M13 is a mesogenic group containing two phenylene radicals which are unsubstituted or substituted by C1-C4-alkyl, methoxy, ethoxy, fluorine, chlorine, bromine, C1-C20-alkoxycarbonyl or C1-C20-alkylcarbonyl and are linked via —O—, —CO—, —CO—O—, —O—CO—O— or —CO—O—; Y16 to Y18 are direct bonds, —O—, —S—, —CO—O—, —O—CO—, —O—CO—-O—, —CO—N(Rc)— or —N(Rc)—CO—; Z13 is a polymerizable group; r3 is 0 or 1; r4 is from 2 to 6; Xc is a chiral group; and Rc is C1-C4-alkyl or hydrogen.
Patent Literature 4 discloses a chiral dopant represented by the formula Z14—Y19-(A14)r5-Y20-M14-Y21-Xd—Y22-(A15)r6-Y23—Z15 wherein the substituents and variables have the following meanings: A14 and A15 are each a spacer with a chain length of 1 to 30 C atoms; Y19 to Y23 are each a chemical bond, —O—, —S—, —C(═O)—, —C(═O)—O—, —O—C(═O)—, —CH═CH—C(═O)—O—, —O—C(═O)—O—, —C(═O)—NRd—, —NRd—C(═O)—, —CH2—O—, —O—CH2—, —CH═N—, —N═CH— or —N═N—; M14 is a mesogenic group; Rd is hydrogen or C1-C4-alkyl; Z14 and Z15 are each hydrogen, C1-C4-alkyl, a polymerizable group or a radical having a polymerizable group; Xd is a dianhydrohexitol residue selected from the group consisting of dianhydrosorbitol, dianhydromannitol and dianhydroiditol; r5 and r6 are 0 or 1; where the radicals Z14, Z15, Y19 to Y23, A14 and A15, can be identical or different, and at least one Z14 or Z15 radical is a polymerizable group or a radical comprising a polymerizable group.
Patent Literature 5 discloses that the isosorbide derivative typified by the compound represented by the following formula 1 is useful as a chiral dopant:
wherein Re and Rf are Ps-Sp-X, F, Cl, Br, I, CN, SCN, SF5, or a straight chain or branched alkyl with 1 to 30 carbon atoms that is unsubstituted, mono- or poly-substituted by F, Cl, Br, I or CN, and wherein one or more non-adjacent CH2 groups are optionally replaced by —O—, —S—, —NH—, —NRg—, —CO—, —COO—, —OCO—, —OCO—O—, —S—CO—, —CO—S—, —CH═CH— or —C≡C— in a manner that O and/or S atoms are not linked directly to one another; Rg is H or alkyl with 1 to 4 carbon atoms; Ps is a polymerizable group; Sp is a spacer group or a single bond; Xe is —O—, —S—, —OCH2—, —CH2O—, —CO—, —COO—, —OCO—, —OCO—O—, —CO—NRg—, —NRg—CO—, —OCH2—, —CH2O—, —SCH2—, —CH2S—, —CH═CH—COO—, —OOC—CH═CH— or a single bond.
However, many of the compounds disclosed in these literatures do not have high helical twisting power.